can't buy apps on Windows computer

It isn't clear to me what it is that you are trying to do.

If you need an app for Bose speakers on an iPhone or iPad, use the App Store app on either of those devices to download it.

If you are trying to use Bose speakers with a Windows 10 computer, that wouldn't involve Apple. Bose support may be able to help. See: https://www.bose.com/en_us/support.html?c=bose_speakers_support

Ability to buy apps by using the iTunes app was removed when iTunes 12.7 was released in 2017.